The return of industry to the utilization of coal as a principal energy source has considerably heightened efforts at developing non-air polluting techniques for its combustion. While coal resources are plentiful, the quality thereof vis-a-vis non polluting combustability varies from region to region. Coal quality, from the standpoint of air quality standards, is predicated principally upon the sulfur and nitrogen content thereof. Concerning the former, generally there are three (3) forms of sulfur in coal: (a) organic sulfur in which the sulfur is covalently bonded to carbon in the general forms --R--S--S--R, and --R--S--R-- or bound as sulfate in the general form R--O--SO.sub.3 ; (b) pyritic sulfur in the form iron pyrite and marcasite, both of which have the same chemical combination, FeS.sub.2, but differ in crystalline structure; and (c) sulfate. Of the above three forms of sulfur, the sulfate content witnessed is very low and of minor importance, while the pyritic sulfur content of coal will range from about 1/2% to 4% and generally is present in an amount exceeding the content of organic sulfur in coal.
Criteria developed under the authority of the Clean Air Act Amendments of 1970 (Pub. L. 91-604, Dec. 31, 1970, 84 Stat. 1676) have provided a limit on the amount of SO.sub.2 permitted in ambient air of 0.03 ppm (80 .mu.g/m.sup.3) as an annual arithmetic mean concentration. A 24 hour maximum limit of 9.14 ppm (365 .mu.g/m.sup.3) not to be exceeded more than once each year also was established. Such criteria can be construed to restrict the combustion of coal at large power plants without scrubbers to utilization of coal which contains somewhere between 0.5% and 1.5% total sulfur. Coal of less than 1% total sulfur generally is considered "low sulfur coal" and coal with greater than about 1.5% to 2.0% total sulfur considered by governmental authorities to be "high" sulfur coal.
The importance of developing a technique for improving coal by lowering the sulfur content thereof prior to combustion becomes apparent in view of estimations that approximately 33% of the coal available in the continental United States exhibits a sulfur content acceptable for combustion without scrubbers. Further in this regard, 62% of the low-sulfur coal reserves in the continental United States are found west of the Mississippi River. Unfortunately, almost 90% of the electric power generating capacity utilizing coal as a heat source is located east of the Mississippi River. See in this regard the following publication:
I Gary, J. H., R. M. Baldwin, C. Y. Bao, M. Kirchner, and J. D. Golden, Research and Develop. Rept. No. 77 prepared for U.S. Office of Coal Research, U.S. Dept. Interior, 99 pp. (1973). PA1 II Colmer, A. R. and M. E. Hinkle. 1947. Role of microorganisms in acid mine drainage: a preliminary report. PA1 III Colmer, A. R., K. L. Temple, and M. E. Hinkle, J. Bacteriol., 59, 317 (1949). PA1 IV Dugan, P. R., "Biochemical Ecology of Water Pollution", pp. 123-137, Plenum Publ. Co., N.Y., N.Y. (1972). PA1 V Duncan, D. W., J. Landesman, and C. C. Walden, Can. J. Microbiol., 13, 397 (1967). PA1 VI Leathan, W. W., S. A. Braley, and L. D. McIntyre, Appl. Microbiol., 1, 61-64. Appl. Microbiol., 1, 65-68 (1953) PA1 VII Temple, K. L. and E. W. Delchamps, App. Microbiol., 1, 225-258 (1953). PA1 VIII Ashmead, D., Colliery Guardian, 190, 694-698 (1955). PA1 IX Silverman, M. P., M. H. Rogoff, and I. Wender, Appl. Microbiol. 9, 491-496 (1961). PA1 X Capes, C. E., A. E. McJLHinney, A. F. Sirianni, and I. E. Puddington, Canadian Mining and Metallurgical Bull., 88-91 (1973). PA1 XI Temple, K. L., and W. A. Kohler, Res. Bull No. 25 Engineering Experiment Station, Univ. of West Virginia, Morgantown (1954). PA1 XII Borichewski, R. M., J. Bacteriol. 93, 597-599 (1967). PA1 XIII Dugan, P. R., Ohio J. Sci., 75, 266 (1975). PA1 XIV Schnaitman, C., and D. G. Lundgren, Can. J. Microbiol., 11, 23 (1965). PA1 XV Tuttle, J. H., P. R. Dugan, C. B., Can. J. Microbiol., 22, 719 (1976). PA1 XVI Tuttle, J. H., P. R. Dugan, and W. A. Apel, Appl. and Environ. Microbiol., 33, 459-469 (1977).
In 1947 bacteria were discovered in acidic coal mine drainage exhibiting a capacity to derive cellular energy requirements from the oxidation of the Fe.sup.+2 portion of iron pyrite. These bacteria ultimately were to be designated as T. ferrooxidans. See in this regard:
Previous to the above publication (II), investigators had known that acidophilic Thiobacilli other than T. ferroxidans, notably T. thiooxidans retained a capability for oxidizing reduced sulfur with concomitant production of sulfuric acid. However, with the identification of T. ferroxidans, investigators developed an interest in its used in connection with the beneficiation of high pyrite coal.
Looking to the oxidation of the chemically reduced compound FeS.sub.2 (pyrite), exposure to oxygen and water results in oxidation of the FeS.sub.2 through a complex series of chemical reactions which are summarized as follows: EQU Fe.sup.+2 .fwdarw.Fe.sup.+3 +electron (1) EQU 2S.sup.-2 +3O.sub.2 +2H.sub.2 O.fwdarw.2(SO.sub.4.sup.-2)+16 electrons+4H.sup.+ ( 2) EQU Sum: FeS.sub.2 +3O.sub.2 +2H.sub.2 O.fwdarw.2H.sub.2 SO.sub.4 +Fe.sup.+3 ( 3)
The oxidized iron (Fe.sup.+3) formed, subsequently reacts with water to produce ferric hydroxide and more acid according to the following equation: EQU Fe.sup.+3 +3H.sub.2 O.fwdarw.Fe(OH).sub.3 +3H.sup.+ ( 4)
In connection with the above reactions, reference is made to publication II above as well as the following additional publications:
As may be expected, prior to the return of interest on part of industry to the utilization of coal as a principal fuel, the concern of investigators was somewhat devoted to control over acidic coal mine drainage, i.e., the formation of sulfuric acid from pyrite. See publication VI above as well as the following:
Subsequent investigations into the activity of acidophilic iron and sulfur oxidizing bacteria in conjunction with the removal of pyrite in coal are described, for example, in the following publication:
The latter publication describes an effort demonstrating that the natural microbial flora of acidic mine waters increases the rate of oxidation of pyrite in a 4% pyrite sulfur sample compared to oxidation in the absence of added bacteria. Subsequent to the above effort, it was reported that Ferrobacillus (presently T. ferrooxidans) accelerated the oxidation of samples of pyrite and coarsely crystalline marcasite extracted from coal (greater than 60% pyrite content) but that the cells were inactive on coarsely crystalline iron pyrite. Oxidation rates in the presence of T. ferrooxidans were increased by reducing the pyrite particle size, while T. thiooxidans cells were found to be inactive on all the pyritic samples examined. See the following publication:
Of course, investigators have for some time engaged in investigations concerning metallurgical leaching where oxidation of sulfide mineral releases a commercially useful metal ion from an ore. The metal ions generally are recovered from the ensuing acid leachate. Typical efforts in this field are described, for example, U.S. Pat. No. 3,455,679. Commercial scale data concerning baceterial removal of pyrite from coal appears in publication X below. In that investigation, various percentages of weathered coal were blended as inoculum of bacteria with run of mine (r.o.m.) coal. The investigators reported that an addition of 10% weathered coal resulted in a reduction in the sulfur content of r.o.m. coal from 6.1% to 2.7% during subsequent agglomeration of pyrite from pulverized coal. A flotation technique was utilized in the investigation.
Investigators have further speculated that the primary role of baceteria in pyrite oxidation is the production of ferric ions and that those ferric ions produced oxidize more pyrite with the comcomitant regeneration of ferrous ions in accordance with the following equation: ##STR1##
This microbiological process is analogous to a chemically catalyzed reaction process. That is, living bacteria are oxidation catalysts promoting the oxidation of insoluble metallic sulfate to soluble sulfate, which is then removed by leaching. The bacteria utilize the pryite nutritionally and grow in the system. Speculation as to the validity of the process of the reaction (5) stemmed from reports, as at publication VI above, that iron oxidizing bacteria are more active than the exclusively sulfur oxidizing bacteria relative to rates of pyrite oxidation, particularly in view of earlier observations that ferric sulfate could oxidize pyrite, as set fourth in the following publication:
From the foregoing, it will be apparent that a microbiological process reducing the sulfur content of coal to an extent permitting its non-polluting combustion might be achieved. However, to the present time no commercially promising technique has been developed. Two characteristics of the acidophilic Thiobacilli derogate from their otherwise advantageous capability for reducing pyritic sulfer. These characteristics are: first, the interval of association of the cultures with coal has been considered too extensive to achieve a commercially practical coal pretreatment process; and, secondly, the acidophilic Thiobacilli are known to produce autotoxic metabolic by-products (principally organic acids) which retard further iron and sulfur oxidation by the cells when present in sufficient concentration. Thus without some correction, the process is self defeating. For further discussion of this autotoxic characteristic, reference is made to the following publications:
In the course of autotoxic activity, lower molecular weight organic acids, particularly alpha-keto acids, some of which are intermediates of the cells' metabolic pathways, inhibit metabolism of iron and sulfur by Thiobacilli by causing the cell membranes thereof to become leaky and ultimately disrupt. As indicated in publication XVI above, this also may allow intolerable amounts of H.sup.+ to enter the cell.